Conventionally the reaction hydraulic turbine consists of spiral casing (or scroll case), stay vane, guide vane, runner and draft tube in that order. Runner also known as impeller is the only rotating passage. Mixed flow hydraulic turbine where the runner inlet flow is radial and runner exit flow is predominantly axial in direction is known as Francis turbine. The runner which is characterized by a large number of blades (usually 11–19), equally spaced circumferentially around the turbine axis; converts hydraulic energy (water head) into mechanical energy. It is called reaction turbine by definition if the pressure drop across the runner alone is more than 60% of the pressure drop occurring across the total water path (spiral casing to draft tube). Francis turbines with various kind of water path are used in practice for a range of specific speed ns, m=70–400 and head range H=50–500 m; or alternatively for a range of unit discharge q11=150–1350 liter/second, and unit speed n11=40–130 rpm. The hydraulic efficiency η (or eta) for prototype runner is expected to be in the range of 90–95%. A runner is designed to be site specific i.e. for a limited range of unit discharge q11: design unit discharge +−100 liters per second (lps) and fixed unit speed n11 based on runner diameter (D or d1), machine speed (n) rpm and rated head H (m); to provide efficient and cavitation free characteristics. The characteristic dimensions are defined as:    Unit speed; n11=(n*D)/√H    Unit discharge; q11=q/(D2√H)    Specific speed; ns,m=n √Pkw/H5/4     Hydraulic efficiency; eta or η=1000*Pkw/(rho*g*q*H)
Where q is discharge in lps (liter/second), Pkw is power developed (in KW) by the turbine, rho (998.2) and g (9.81) are water density and acceleration due to gravity.
A typical turbine characteristics are plotted as shown in FIG. 1A of the accompanying drawings showing efficiency iso-contours e1, e2 . . . optimum guide vane openings a1, a2 . . . as function of unit discharge q11 and unit speed n11. Note:    Guide vane openings: a1<a2<a3<a4 . . .    Hydraulic efficiency (eta); e1>e2>e3>e4 . . .For a unit design speed, say n11,d a turbine is designed to deliver q11,d at operative point ‘O’ (FIG. 1a). Usually the turbine is expected to operate around print ‘O’ within q11,d±100 lps efficiently. With the years of operation, the head deteriorates significantly, hence operating point shift to lesser efficient location say R. The point R is at higher discharge and at slightly higher n11 at lower efficiency point, resulting into lesser turbine power. New hydro power plant needs huge investment hence designer seeks for renovation and modernization of existing plant with minimum investment. Retrofitting runner is an obvious cost-effective first choice. The aim of the invention is to develop a new runner which has got optimum efficiency near new operating point R (FIG. 1B), and suit existing water path and retaining stationary components of hydraulic paths (viz. Spiral casing, guide vane, draft tube and stay vane).
George E. Hecker & Willem Jansen have described turbine having 2 or more runner blades each having a Cork Screw configuration (U.S. Pat. No. 5,997,242; Dec. 7, 1999). One wicket gate configuration for hydraulic turbine is proposed by A Gokhman (U.S. Pat. No. 5,441,384; Aug. 15, 1995). David G. Homes et. al has proposed blade configuration for Francis runner for improved cavitation-free performance (U.S. Pat. No. 4,379,757; Oct. 30, 1984). Benno Buchelt has proposed a blade for Kaplan turbine (U.S. Pat. No. 6,007,297, Dec. 28, 1999). The reported work by Kurokawa et al. (U.S. Pat. No. 6,217,285 B1; Apr. 17, 2001) and Strycek et al. (U.S. Pat. No. 3,964,841; Jun. 22, 1976) concerning the design of centrifugal blower, fan and pump for flow discharge as output. They are not meant for turbine application where power is output. The work by Billdal et al. (U.S. Pat. No. 6,135,716; Oct. 24, 2000) proposed impeller for turbine with leaning of leading and trailing edges with respect to hub and shroud, respectively, in the direction of rotation. Harada et al. (U.S. Pat. No. 6,338,610 B1; Jan. 15, 2002) propose impeller for discharge as output with secondary flow loss reduction as goal by providing leaned blade. Nishikawa (U.S. Pat. No. 4,274,810; Jun. 23, 1981) proposes blade for fan application. Centrifugal compressor for discharge as output is proposed by Seleznev et al. (U.S. Pat. No. 3,973,872; Aug. 10, 1976). Swearingen (U.S. Pat. No. 3,610,775; Oct. 5, 1971) is for impeller to work with flow motion in gaseous medium entrained with liquid and solid particles. Kugel (U.S. Pat. No. 2,042,064; May 26, 1936), Kaplan (U.S. Pat. No. 1,509,653; Sep. 23, 1924) and Konda M. (U.S. Pat. No. 3,440,969; Apr. 29, 1969) deal with various constructional features of runners for centrifugal flow machines. The present invention relates to design of new runner blade consisting of blade profiles with reduced plan angle for retrofit work to existing mixed flow machines Francis turbine) water paths, to suit deteriorated head.